Method for antenna calibration and active antenna system for use in antenna calibration

ABSTRACT

A method for antenna calibration for an active antenna system is disclosed. According to an embodiment, test signals are generated for multiple antennas of the active antenna system. The test signals are transmitted via the multiple antennas. A first signal that results from the transmission of the test signals is received over the air. A second signal is received from a coupler network of the active antenna system. The coupler network is configured to generate coupled signals of the test signals and combine the coupled signals into the second signal. Calibration information for compensating an influence of the coupler network is determined based on the first and second signals. An active antenna system is also disclosed for use in antenna calibration.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 17/291,104 filed on May4, 2021 (status pending), which is the 35 U.S.C. § 371 National Stage ofInternational Patent Application No. PCT/CN2018/114005, filed on Nov. 5,2018. The above identified applications are incorporated by thisreference.

TECHNICAL FIELD

Embodiments of the disclosure generally relate to wirelesscommunication, and, more particularly, to a method for antennacalibration and an active antenna system for use in antenna calibration.

BACKGROUND

This section introduces aspects that may facilitate better understandingof the present disclosure. Accordingly, the statements of this sectionare to be read in this light and are not to be understood as admissionsabout what is in the prior art or what is not in the prior art.

Massive multiple-input multiple-output (MIMO), as a key technology inthe 5th generation (5G) communication technology, can greatly improvethe cell coverage and capacity by employing a large number of antennasat base station. Beamforming is one realization and widely used inactive antenna system (AAS). To guarantee effective beamforming, antennacalibration (AC) is required to achieve good phase alignment among radiochannels. The phase is defined from baseband to antenna reference point(ARP). However, AC loop is usually not only from baseband to ARP, butalso includes the feedback path, i.e. from ARP to antenna interfacetransceiver (AI TRX). The phase difference between ARP and AI TRX needsto be compensated during each AC event. For this reason, coupler networkcalibration is necessary.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the detaileddescription. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

One of the objects of the disclosure is to provide an improved solutionfor antenna calibration.

According to one aspect of the disclosure, there is provided a methodfor antenna calibration for an active antenna system. The methodcomprises generating test signals for multiple antennas of the activeantenna system. The method further comprises transmitting the testsignals via the multiple antennas. The method further comprisesreceiving, over the air, a first signal that results from thetransmission of the test signals. The method further comprises receivinga second signal from a coupler network of the active antenna system thatis configured to generate coupled signals of the test signals andcombine the coupled signals into the second signal. The method furthercomprises determining calibration information for compensating aninfluence of the coupler network, based on the first and second signals.

In an embodiment of the disclosure, the test signals carry trainingsequences.

In an embodiment of the disclosure, the test signals are generated basedon multi-carrier code division multiple access (MC-CDMA).

In an embodiment of the disclosure, generating the test signalscomprises generating a root sequence. Generating the test signalsfurther comprises generating spreading codes for the multiple antennas.Generating the test signals further comprises calculating, for each ofthe multiple antennas, a product between the root sequence and one ofthe spreading codes.

In an embodiment of the disclosure, the root sequence is stored in acommon memory shared between the multiple antennas.

In an embodiment of the disclosure, the generating of the root sequence,the generating of the spreading codes and the calculating of theproducts are performed by the active antenna system.

In an embodiment of the disclosure, generating the root sequencecomprises generating an initial root sequence in frequency domain.Generating the root sequence further comprises transforming the initialroot sequence into the root sequence by inverse fast Fouriertransformation (IFFT).

In an embodiment of the disclosure, the initial root sequence is apseudo noise sequence.

In an embodiment of the disclosure, the pseudo noise sequence is one of:a Zadoff-Chu sequence; an M-sequence; and a Gold sequence.

In an embodiment of the disclosure, the spreading codes are generated byusing one of: Hadamard matrix and Walsh matrix.

In an embodiment of the disclosure, the test signals are generated forthe multiple antennas simultaneously. The test signals are transmittedvia the multiple antennas simultaneously.

In an embodiment of the disclosure, the multiple antennas are dividedinto subgroups. The generating of the test signals, the transmitting ofthe test signals, the receiving of the first signal and the receiving ofthe second signal are performed for each of the subgroups respectively.

In an embodiment of the disclosure, the subgroups include commonsubgroups and one additional subgroup. A union set of the commonsubgroups is a set of the multiple antennas and an intersection betweenany two of the common subgroups is an empty set. The one additionalsubgroup includes, for each subgroup in the common subgroups, a memberfrom the subgroup.

In an embodiment of the disclosure, determining the calibrationinformation comprises obtaining first inphase and quadrature (IQ) datafrom the first signal. Determining the calibration information furthercomprises obtaining second IQ data from the second signal. Determiningthe calibration information further comprises determining, as thecalibration information, a phase difference between the first and secondIQ data.

According to another aspect of the disclosure, there is provided anactive antenna system. The active antenna system comprises a digitalsignal generator configured to generate digital signals for multipleantennas. The active antenna system further comprises multipletransmitters configured to process the digital signals into test signalsfor transmission via the multiple antennas. The active antenna systemfurther comprises the multiple antennas configured to transmit the testsignals. The active antenna system further comprises a coupler networkconnected between the multiple transmitters and the multiple antennasand configured to generate coupled signals of the test signals andcombine the coupled signals into a feedback signal. The active antennasystem further comprises a feedback receiver configured to receive thefeedback signal.

In an embodiment of the disclosure, the digital signals are trainingsequences.

In an embodiment of the disclosure, the digital signal generator isconfigured to generate the digital signals based on MC-CDMA.

In an embodiment of the disclosure, the digital signal generatorcomprises a root sequence generator configured to generate a rootsequence. The digital signal generator further comprises a spreadingcode generator configured to generate spreading codes for the multipleantennas. The digital signal generator further comprises amultiplication unit configured to calculate, for each of the multipleantennas, a product between the root sequence and one of the spreadingcodes.

In an embodiment of the disclosure, the root sequence is stored in acommon memory shared between the multiple antennas.

In an embodiment of the disclosure, the root sequence generator isconfigured to generate the root sequence by generating an initial rootsequence in frequency domain and transforming the initial root sequenceinto the root sequence by IFFT.

In an embodiment of the disclosure, the digital signal generator isconfigured to generate the digital signals for the multiple antennassimultaneously.

In an embodiment of the disclosure, the multiple antennas are dividedinto subgroups. The digital signal generator is configured to generatethe digital signals for each of the subgroups respectively.

In an embodiment of the disclosure, the subgroups include commonsubgroups and one additional subgroup. A union set of the commonsubgroups is a set of the multiple antennas and an intersection betweenany two of the common subgroups is an empty set. The one additionalsubgroup includes, for each subgroup in the common subgroups, a memberfrom the subgroup.

According to some embodiment(s) of the disclosure, the calibrationaccuracy can be improved since the calibration configuration is closelysame with practical use.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the disclosure willbecome apparent from the following detailed description of illustrativeembodiments thereof, which are to be read in connection with theaccompanying drawings.

FIG. 1 is a diagram illustrating the existing solution for antennacalibration;

FIG. 2 is a flowchart illustrating a method for antenna calibrationaccording to an embodiment of the disclosure;

FIG. 3 is a flowchart for explaining the method of FIG. 2 ;

FIG. 4 is a flowchart for explaining the method of FIG. 3 ;

FIG. 5 shows an exemplary example for explaining the method of FIG. 3 ;

FIG. 6 is a flowchart for explaining the method of FIG. 2 ;

FIG. 7 shows an exemplary example for explaining the method of FIG. 2 ;

FIG. 8 shows the calibration result for the example of FIG. 7 ; and

FIG. 9 is a block diagram showing an active antenna system according toan embodiment of the disclosure.

DETAILED DESCRIPTION

For the purpose of explanation, details are set forth in the followingdescription in order to provide a thorough understanding of theembodiments disclosed. It is apparent, however, to those skilled in theart that the embodiments may be implemented without these specificdetails or with an equivalent arrangement.

Currently, there are two commonly-used calibration methods. One is nocalibration but guaranteed by hardware (HW). This puts stringentrequirement on radio distribution network board (RDNB)/antenna filterunit (AFU) production. That is, HW must guarantee coherent phase ofantenna calibration (AC) loopback path for each branch, leading tohigh-cost and difficulty in implementation. The other one is cascadingcalibration method, which needs to calibrate the S parameter of RDNB/AFUand AC cable separately and then cascade them. FIG. 1 shows a schematicillustration for RDNB/AFU calibration. As shown, an exemplary example ofN=64 is provided, where N is the number of branches which are to becalibrated. The RDNB 102 is mounted by the fixtures 1041 and 1042. Theinput side of the RDNB 102 is connected with the switch box 1061 via 64cables and the output side of the RDNB 102 is connected with the switchbox 1062 via 64 cables. A test signal is generated by the vector networkanalyzer (VNA) 108 and input into the switch box 1061. Then, an outputsignal from the switch box 1062 is received by the VNA 108. A feedbacksignal from the RDNB 102 (specifically, the “SUM” unit) is also receivedby the VNA 108. Therefore, this method needs complicated assembling,fixture, switch box, VNA, over 2*N cables and extra connectors, which iscostly and complicated to setup. Moreover, the complicated testbenchalso needs to be calibrated before coupler network calibration.Calibration error caused by the cascading method itself and wrongoperation by operators in factory is also unavoidable.

The present disclosure proposes an improved solution for antennacalibration. Hereinafter, the solution will be described in detail withreference to FIGS. 2-9 .

FIG. 2 is a flowchart illustrating a method for antenna calibrationaccording to an embodiment of the disclosure. The method may beapplicable to an active antenna system comprising multiple antennas anda coupler network (also referred to as RDNB or AFU). At block 202, testsignals are generated for the multiple antennas of the active antennasystem. For example, the test signals may be generated based onmulti-carrier code division multiple access (MC-CDMA). In the case thatthe test signals carry training sequences, the generation based onMC-CDMA may include blocks 312-316 of FIG. 3 .

At block 312, a root sequence is generated. Since MC-CDMA is used, theroot sequence is generated based on orthogonal frequency divisionmultiplexing (OFDM), where one root sequence may be equivalent to oneOFDM symbol. For example, block 312 may be implemented as blocks 312-1and 312-2 as shown in FIG. 4 . At block 312-1, an initial root sequenceis generated in frequency domain. The initial root sequence may be anypseudo noise sequence such as Zadoff-Chu (ZC) sequence, M-sequence, Goldsequence, or the like. In the case that ZC sequence is used, a lowpeak-to-average ratio (PAR) root sequence may be generated. At block312-2, the initial root sequence is transformed into the root sequenceby inverse fast Fourier transformation (IFFT).

At block 314, spreading codes are generated for the multiple antennas.Since MC-CDMA is used, all of the spreading codes are orthogonal to eachother. Various techniques for generation of orthogonal spreading codesmay be used, such as using Hadamard matrix, Walsh matrix, or the like.In the case that Hadamard matrix is used, no complex multiplication isneeded. Different antennas may have different spreading codes. Supposethe number of the multiple antennas (or antenna branches) is N. Then,for each antenna, the corresponding spreading code has N components. Atblock 316, a product between the root sequence and one of the spreadingcodes is calculated for each of the multiple antennas. In the aboveexample of N antennas, for each antenna, the root sequence (equivalentto one OFDM symbol) may be first repeated to N root sequences(equivalent to N OFDM symbols) and then multiplied with thecorresponding spreading code having N components.

Thus, the generation of the training sequences is divided into twoparts. The first part generates the root sequence, which is the same toall of the multiple antennas. The second part exploits direct-spreadingto the root sequence (equivalent to an OFDM symbol) to generate MC-CDMAsymbols. Then, the resulting products (training sequences) at block 316may undergo further processing to become the test signals fortransmission via the multiple antennas. The further processing may beperformed by the transceiver array of the active antenna system, whichmay be well known to those skilled in the art.

As a first option, blocks 312-316 may be performed by a hardware circuit(such as integrated circuit, field programmable gate array (FPGA), orthe like) in the active antenna system. For example, the hardwarecircuit may be part of a digital radio component of the active antennasystem. For the first option, the root sequence may be stored in acommon memory shared between the multiple antennas. The common memory isa memory which is usually used in an active antenna system to savememory size. Thus, the size of the common memory is small. Since theroot sequence is the same to all antenna branches and stored in thecommon memory, the limitation on the size of the common memory can beovercome. As a second option, blocks 312-316 may be performed by adedicated hardware circuit separate from the active antenna system. Inthis case, the dedicated hardware circuit needs to be connected with theactive antenna system to provide the generated digital signals (theproducts at block 316) to the active antenna system. This leads torelatively higher cost than the first option. Although the test signalsare described as carrying training sequences hereinabove, the presentdisclosure is not limited to training signals and any other suitabletest signals may be used instead.

FIG. 5 shows an exemplary example for explaining the method of FIG. 3 .In this exemplary example, suppose the number of antenna branches(denoted by N) is 64, the sampling rate is 122.88 Mhz, and thesubcarrier spacing is 960 Khz. Then, the length of the root sequence(denoted by N_(s)) can be calculated as: 122.88 Mhz/960 Khz=128. Asshown in FIG. 5 , the spreading codes are generated as a 64*64orthogonal matrix which is a Hadamard matrix. The root sequence (a ZCsequence in this example) having the length of 128 samples is repeatedinto N_(eq)=64 blocks, which constitute a column vector. For the firstbranch Br0, the column vector is multiplied by the first row of theorthogonal matrix, resulting in 8192 samples. Similarly, for the i-thbranch (where 2≤i≤64), the column vector is multiplied by the i-th rowof the orthogonal matrix. Thus, the total length of one sequence to onebranch is N_(s)*N_(eq)=8192. Thus, the maximum size for the commonmemory is only 128 samples. The test signals resulting from the trainingsequences in this example have similar power spectrum density (PSD) as100M new radio (NR) signal, but with lower PAR.

At block 204, the test signals are transmitted via the multipleantennas. For example, the test signals may be transmitted by the activeantenna system in an anechoic chamber to perform an over-the-air (OTA)test. The active antenna system may be placed on a testbench. Since thetest signals are transmitted over the air, the calibration configurationis closely same with practical use. Compared with the existing cascadingcalibration method, extra assembling and connectors are not required,leading to convenient testbench setup and low cost. At block 206, afirst signal that results from the transmission of the test signals isreceived over the air. A signal analyzer may be placed in the anechoicchamber to receive the first signal. At block 208, a second signal isreceived from a coupler network of the active antenna system. Thecoupler network is configured to generate coupled signals of the testsignals and combine the coupled signals into the second signal. Thesecond signal may be received by a feedback receiver (e.g. in an antennainterface transceiver) of the active antenna system.

At block 210, calibration information for compensating an influence ofthe coupler network is determined based on the first and second signals.For example, block 210 may be implemented as blocks 618-622 of FIG. 6 .At block 618, first inphase and quadrature (IQ) data is obtained fromthe first signal. Block 618 may be performed by the signal analyzer. Theobtained first IQ data may be stored in a memory of the signal analyzer.At block 620, second IQ data is obtained from the second signal. Block620 may be performed by the feedback receiver. The obtained second IQdata may be stored in a memory (e.g. Ethernet test access point, simplyreferred to as eTAP) of the active antenna system.

At block 622, a phase difference between the first and second IQ data isdetermined as the calibration information. As an exemplary example,block 622 may be implemented as the following sub-blocks. At the firstsub-block, de-spreading is performed. Assume the signal received by thesignal analyzer or the feedback receiver (denoted by w) can be expressedas: w=[w₁(n), w₂(n), . . . , w_(N)(n)], where N is the number of antennabranches and n is the index number of a sample. Then, the receivedsymbol of branch m can be expressed as:

{circumflex over (x)} ^((m))(n)=Σ_(l=1) ^(N) w _(l)(n)H(m,l),

where H is a N×N Hadamard matrix. At the second sub-block,synchronization is performed. Specifically, cross-correlation xcorrbetween a reference symbol x(n) and the received symbol {circumflex over(x)}^((m))(n) is performed, which can be expressed as:

c=xcorr({circumflex over (x)} ^((m))(n),x(n)),

where the peak of c indicates the start point of symbol.

At the third sub-block, FFT is applied to the received symbol, which canbe expressed as:

${{{\overset{\hat{}}{X}}^{(m)}(k)} = {\sum_{n = 0}^{N_{s} - 1}{{{\overset{\hat{}}{x}}^{(m)}(n)}{\exp\left( {{- j}\frac{2\pi nk}{N_{s}}} \right)}}}},{k = 0},\ldots,N_{s},$

where N_(s) is the number of samples in one OFDM symbol. At the fourthsub-block, the phase at active subcarriers is estimated. Note that othersubcarriers are zero. This can be expressed as:

${\overset{˜}{\theta}\left( {m,k} \right)} = \left\{ {\begin{matrix}{\frac{{\overset{\hat{}}{x}}^{(m)}(k)}{X(k)}\ ,\ {k \in \ {{active}\ {subcarriers}}}} \\{0,\ {otherwise}}\end{matrix},} \right.$

where X(k) is the root sequence which is a ZC sequence. At the fifthsub-block, p-order polynomial fitting is utilized to smooth the curve.The estimated phase can be expressed as:

θ(m,k)=a ₀ +a ₁ k+ . . . +a _(p) k ^(p)≈{tilde over (θ)}(m,k).

Then, the above first to fifth sub-blocks are repeated to get phaseestimation θ(m,k) for all branches. The difference between the twocaptured data is the wanted distortion caused by the coupler network.The resultant delay and phase may be stored into the database of theactive antenna system for future use.

Blocks 202-208 may be performed in various ways. As an option, the testsignals may be generated for the multiple antennas simultaneously.Correspondingly, the test signals may be transmitted via the multipleantennas simultaneously. The receiving of the first signal at block 206and the receiving of the second signal at block 208 may also beperformed for the multiple antennas simultaneously. In this way, areference clock is not necessary to be connected between the signalanalyzer and the feedback receiver since all of the multiple antennasare calibrated at the same time.

As another option, the multiple antennas may be divided into subgroups.Blocks 202-208 (i.e. the generating of the test signals, thetransmitting of the test signals, the receiving of the first signal andthe receiving of the second signal) may be performed for each of thesubgroups respectively. As an example for this option, the subgroups mayinclude common subgroups and one additional subgroup. A union set of thecommon subgroups is a set of the multiple antennas and an intersectionbetween any two of the common subgroups is an empty set. The oneadditional subgroup includes, for each subgroup in the common subgroups,a member from the subgroup. The one additional subgroup may be used as areference subgroup to compensate the phase drift occurring due to thesignal analyzer and the feedback receiver when calibration is performedfor different common subgroups. Therefore, it should be noted that twoblocks shown in succession in the figures may, in fact, be executedsubstantially concurrently, or the blocks may sometimes be executed inthe reverse order, depending upon the functionality involved.

FIG. 7 shows an exemplary example for explaining the method of FIG. 2 .In this exemplary example, the same configuration as that described withrespect to FIG. 5 is used. As shown, an active antenna system (AAS)product includes a transceiver board, a coupler network and an antennaarray which are integrated together. The AAS product is placed in an OTAchamber to perform an OTA test. The root sequence is stored in a commonmemory of a digital data source (DDS) when it has been generated by theDDS for the first time. The DDS may be provided in the digital radioASIC/FPGA of the AAS product. The root sequence is sent to each antennabranch and multiplied by a corresponding spreading code to generateorthogonal sequences. The generated orthogonal sequences are processedinto test signals by a transceiver array (not shown) provided on thetransceiver board.

After the test signals are sent to the antenna reference point (ARP),two receivers are enabled for signal capture simultaneously. The firstreceiver is a signal analyzer (SA) with antenna, which retrieves signalin the manner of OTA. The SA may use an IQ analyzer to capture and storeIQ data in its internal memory. The feedback receiver is an antennainterface transceiver (AI TRX), which retrieves signal from the couplernetwork. The AI TRX may use a random access memory (RAM) to capture andstore IQ data. Since the AI TRX and the RAM is also used in AC function,this configuration does not need extra HW expense. The IQ data stored inthe SA and the RAM may be read by wireless communication and undergoprocessing to determine the phase difference. In this way, the wholeprocess may be automated, avoiding the error caused by improperoperation.

As an option, signals on all branches may be triggered simultaneously,and captured in the two receivers simultaneously. Thus, a referenceclock is not necessary to be connected between the SA and the AI TRXsince all 64 branches are calibrated at the same time. As anotheroption, if it is difficult to trigger all branches at the same time(depending on HW implementation), N branches may be divided into Msub-calibration groups (SCGs). Each SCG contains N/M branches, where N/Mis an integer. The N/M branches in one SCG may be calibratedsimultaneously. The calibration may be performed over M times to extendto all branches. In this case, the phase drift due to the SA and the AITRX should be considered. Otherwise, the result will include wrongadditional phase between the SA and the AI TRX. To overcome this issue,in the M times of measurement, one additional SCG may be utilized, whichincludes one branch from each of the original M SCGs. And this specialbranch picked from the original SCGs may be used as a reference branch,and taken into calculation. Thus, there may be totally (M+1) SCGs.

FIG. 8 shows the calibration result for the example of FIG. 7 . Thecalibration result was obtained by using signal processing to analyzethe phase difference of N branches at the ARP and the AI TRX. Bycomparing the captured IQ data with a reference signal (e.g. the rootsequence), the phase of 64 branches for the SA and the AI TRX wasestimated respectively. The dotted line is the phase estimation for theIQ data from the SA and the solid line is for the IQ data from the RAMof the AI TRX. The difference of these two lines is the wanted couplercalibration results for 64 branches. The resultant phase difference maybe stored in a database and will be used in AC algorithm to compensatephase mismatch between the branches due to the coupler network.

FIG. 9 is a block diagram showing an active antenna system according toan embodiment of the disclosure. As shown, the active antenna system 900comprises a digital signal generator 902, multiple transmitters 904,multiple antennas 906, a coupler network 908 and a feedback receiver910. The digital signal generator 902 is configured to generate digitalsignals for the multiple antennas 906, as described above with respectto blocks 312-316 of FIG. 3 . The multiple transmitters 904 areconfigured to process the digital signals into test signals fortransmission via the multiple antennas 906. The multiple transmitters904 may be the transmission portion of a transceiver array for use invarious AAS products. The multiple antennas 906 are configured totransmit the test signals. The coupler network 908 is connected betweenthe multiple transmitters 904 and the multiple antennas 906 andconfigured to generate coupled signals of the test signals and combinethe coupled signals into a feedback signal. The coupler network 908 mayalso be referred to as RDNB or AFU. The feedback receiver 910 isconfigured to receive the feedback signal.

As an example, the digital signal generator 902 may include a rootsequence generator 9022, a spreading code generator 9024 and amultiplication unit 9026. The root sequence generator 9022 may beconfigured to generate a root sequence, as described above with respectto block 312 of FIG. 3 . The spreading code generator 9024 may beconfigured to generate spreading codes for the multiple antennas 906, asdescribed above with respect to block 314 of FIG. 3 . The multiplicationunit 9026 may be configured to calculate, for each of the multipleantennas 906, a product between the root sequence and one of thespreading codes, as described above with respect to block 316 of FIG. 3.

In general, the various exemplary embodiments may be implemented inhardware or special purpose circuits, software, logic or any combinationthereof. For example, some aspects may be implemented in hardware, whileother aspects may be implemented in firmware or software which may beexecuted by a controller, microprocessor or other computing device,although the disclosure is not limited thereto. While various aspects ofthe exemplary embodiments of this disclosure may be illustrated anddescribed as block diagrams, flow charts, or using some other pictorialrepresentation, it is well understood that these blocks, apparatus,systems, techniques or methods described herein may be implemented in,as non-limiting examples, hardware, software, firmware, special purposecircuits or logic, general purpose hardware or controller or othercomputing devices, or some combination thereof.

As such, it should be appreciated that at least some aspects of theexemplary embodiments of the disclosure may be practiced in variouscomponents such as integrated circuit chips and modules. It should thusbe appreciated that the exemplary embodiments of this disclosure may berealized in an apparatus that is embodied as an integrated circuit,where the integrated circuit may comprise circuitry (as well as possiblyfirmware) for embodying at least one or more of a data processor, adigital signal processor, baseband circuitry and radio frequencycircuitry that are configurable so as to operate in accordance with theexemplary embodiments of this disclosure.

It should be appreciated that at least some aspects of the exemplaryembodiments of the disclosure may be embodied in computer-executableinstructions, such as in one or more program modules, executed by one ormore computers or other devices. Generally, program modules includeroutines, programs, objects, components, data structures, etc. thatperform particular tasks or implement particular abstract data typeswhen executed by a processor in a computer or other device. The computerexecutable instructions may be stored on a computer readable medium suchas a hard disk, optical disk, removable storage media, solid statememory, RAM, etc. As will be appreciated by one of skill in the art, thefunction of the program modules may be combined or distributed asdesired in various embodiments. In addition, the function may beembodied in whole or in part in firmware or hardware equivalents such asintegrated circuits, field programmable gate arrays (FPGA), and thelike.

References in the present disclosure to “one embodiment”, “anembodiment” and so on, indicate that the embodiment described mayinclude a particular feature, structure, or characteristic, but it isnot necessary that every embodiment includes the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to implement such feature, structure, orcharacteristic in connection with other embodiments whether or notexplicitly described.

It should be understood that, although the terms “first”, “second” andso on may be used herein to describe various elements, these elementsshould not be limited by these terms. These terms are only used todistinguish one element from another. For example, a first element couldbe termed a second element, and similarly, a second element could betermed a first element, without departing from the scope of thedisclosure. As used herein, the term “and/or” includes any and allcombinations of one or more of the associated listed terms.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to limit the present disclosure. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”,“comprising”, “has”, “having”, “includes” and/or “including”, when usedherein, specify the presence of stated features, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, elements, components and/or combinations thereof. Theterms “connect”, “connects”, “connecting” and/or “connected” used hereincover the direct and/or indirect connection between two elements.

The present disclosure includes any novel feature or combination offeatures disclosed herein either explicitly or any generalizationthereof. Various modifications and adaptations to the foregoingexemplary embodiments of this disclosure may become apparent to thoseskilled in the relevant arts in view of the foregoing description, whenread in conjunction with the accompanying drawings. However, any and allmodifications will still fall within the scope of the non-Limiting andexemplary embodiments of this disclosure.

1. A method for antenna calibration for an active antenna system, themethod comprising: generating test signals for a plurality of antennasof the active antenna system; transmitting the test signals via theplurality of antennas; receiving, over the air, a first signal thatresults from the transmission of the test signals; receiving a secondsignal from a coupler network of the active antenna system that isconfigured to generate coupled signals of the test signals and combinethe coupled signals into the second signal; and determining calibrationinformation for compensating an influence of the coupler network, basedon the first and second signals.
 2. The method of claim 1, whereingenerating the test signals comprises: generating a root sequence;generating spreading codes for the plurality of antennas; andcalculating, for each of the plurality of antennas, a product betweenthe root sequence and one of the spreading codes.
 3. The method of claim2, wherein the generating of the root sequence, the generating of thespreading codes and the calculating of the products are performed by theactive antenna system.
 4. The method of claim 2, wherein generating theroot sequence comprises: generating an initial root sequence infrequency domain; and transforming the initial root sequence into theroot sequence by inverse fast Fourier transformation, IFFT.
 5. Themethod of claim 4, wherein the initial root sequence is a pseudo noisesequence.
 6. The method of claim 5, wherein the pseudo noise sequence isone of: a Zadoff-Chu sequence; an M-sequence; and a Gold sequence. 7.The method of claim 2, wherein the spreading codes are generated byusing one of: Hadamard matrix and Walsh matrix.
 8. The method of claim1, wherein the test signals are generated for the plurality of antennassimultaneously; and wherein the test signals are transmitted via theplurality of antennas simultaneously.
 9. The method claim 1, wherein theplurality of antennas are divided into subgroups; and wherein thegenerating of the test signals, the transmitting of the test signals,the receiving of the first signal and the receiving of the second signalare performed for each of the subgroups respectively.
 10. The method ofclaim 9, wherein the subgroups include common subgroups and oneadditional subgroup; wherein a union set of the common subgroups is aset of the plurality of antennas and wherein an intersection between anytwo of the common subgroups is an empty set; and wherein the oneadditional subgroup includes, for each subgroup in the common subgroups,a member from the subgroup.
 11. The method of claim 1, whereindetermining the calibration information comprises: obtaining firstinphase and quadrature, IQ, data from the first signal; obtaining secondIQ data from the second signal; and determining, as the calibrationinformation, a phase difference between the first and second IQ data.12. An active antenna system comprising: a plurality of antennas; adigital signal generator configured to generate digital signals for theplurality of antennas; a plurality of transmitters configured to processthe digital signals into test signals for transmission via the pluralityof antennas; a coupler network connected between the plurality oftransmitters and the plurality of antennas and configured to generatecoupled signals of the test signals and combine the coupled signals intoa feedback signal; and a feedback receiver configured to receive thefeedback signal.
 13. The active antenna system of claim 12, wherein thedigital signals are training sequences.
 14. The active antenna system ofclaim 12, wherein the digital signal generator is configured to generatethe digital signals based on multi-carrier code division multipleaccess.
 15. The active antenna system of claim 14, wherein the digitalsignal generator comprises: a root sequence generator configured togenerate a root sequence; a spreading code generator configured togenerate spreading codes for the plurality of antennas; and amultiplication unit configured to calculate, for each of the pluralityof antennas, a product between the root sequence and one of thespreading codes.
 16. The active antenna system of claim 15, wherein theroot sequence is stored in a common memory shared between the pluralityof antennas.
 17. The active antenna system of claim 15, wherein the rootsequence generator is configured to generate the root sequence by:generating an initial root sequence in frequency domain; andtransforming the initial root sequence into the root sequence by inversefast Fourier transformation, IFFT.
 18. The active antenna system ofclaim 12, wherein the digital signal generator is configured to generatethe digital signals for the plurality of antennas simultaneously. 19.The active antenna system of claim 12, wherein the plurality of antennasare divided into subgroups; and wherein the digital signal generator isconfigured to generate the digital signals for each of the subgroupsrespectively.
 20. The active antenna system of claim 19, wherein thesubgroups include common subgroups and one additional subgroup; whereina union set of the common subgroups is a set of the plurality ofantennas and wherein an intersection between any two of the commonsubgroups is an empty set; and wherein the one additional subgroupincludes, for each subgroup in the common subgroups, a member from thesubgroup.